This invention relates to an energy-efficient control method for a variable capacity refrigerant compressor of an air conditioning system.
Variable capacity refrigerant compressors have been utilized in both manual and automatic vehicle air conditioning systems, primarily to reduce engine load disturbances associated with compressor clutch cycling. In a typical implementation, the compressor includes one or more pistons coupled to a tiltable wobble plate or swash plate, and a pneumatic or electromagnetic control valve for adjusting the pressure in a crankcase of the compressor to control the compressor capacity. The system control strategy usually involves adjusting the compressor capacity to maintain a predetermined low-side refrigerant condition (refrigerant suction pressure or evaporator outlet air temperature, for example) that provides maximum cooling without evaporator icing, and using a high-side pressure switch to disengage the compressor clutch if the refrigerant discharge pressure becomes too high. The inlet air may consist of outside air or recirculated cabin air, and the temperature of the discharge air is typically controlled by adjusting a mechanism (such as an air mix door) that reheats a portion of the conditioned air.
While the above-described control strategy is simple and reasonably effective, it has been recognized that the energy efficiency of the system could be significantly improved by increasing the usage of recirculated cabin air and reducing the compressor capacity in a way that provides adequate dehumidification while minimizing reheating of the conditioned air. However, since energy-efficient controls typically require a number of external sensors for measuring system and ambient parameters, development efforts have primarily been focused on high-end automatic climate control systems that usually include such sensors anyway. Accordingly, what is needed is a compressor capacity control method that provides energy-efficient operation at a low cost.
The present invention is directed to an improved and energy-efficient control method for a variable capacity refrigerant compressor of an air conditioning system, where the control is based on the compressor suction and discharge pressures and a measure of the ambient temperature. According to the invention, a target suction pressure is selected based on the ambient temperature and the sensed discharge pressure, and the capacity of the compressor is adjusted as required to attain the target suction pressure. In a first embodiment, the ambient temperature is used to select a target evaporator outlet air temperature, which is used along with the sensed discharge pressure to select the target suction pressure, while in a second embodiment the target suction pressure is selected directly on the basis of the ambient temperature and the sensed discharge pressure. The control method is conveniently and cost-effectively carried out in an implementation where the compressor includes a capacity control valve with integral suction and discharge pressure sensors, and integral signal conditioning circuitry.